1. Field of the Invention
This invention relates to a pickup head of optical disk drive, and more particularly to a feedforward compensation control device by utilizing acceleration and a feedback control device for position to allow the pickup head to be able to move smoothly and obey the designed velocity profile and reach the destination to complete the tracking control easily.
2. Description of Related Art
Due to the continuous promotion of the information technology, the conventional Disks have been gradually replaced by optical disk drive because of their developing technology being limited and the storing capability being not large enough. The optical disk drive has advantages of high capability of storage for storing a large quantity of information and further being able to execute read/write for the present products so that the optical disk drive has been popularly accepted and applied by the users.
However, during the production of the optical disk drive, the part devices including sled motor and transmission device are not able to be made exactly the same and the characteristics of the electronic elements like resistors and capacitors are changing during being in operation. On the other hand, during being in operation, the environmental temperature does also affect the electronic devices, which are used in optical disk drive. Moreover, because of the difference of the optical characteristics of photo sensor and the reflectivity of the optical disk itself, a proper adjustment is needed to avoid the focusing problem and the seek error. FIG. 1 illustrates the structure of the pickup head of an optical disk drive. In the technology of reading the information stored in the optical disk drive, the operation of seek is to move the pickup head to the right position on the track of information. Referring to FIG. 1, the pickup head includes a lens 10, which further includes a laser diode, a photo sensor and other related elements, a sled motor 11 and a tracking coil 12. The operation of seek is done through both the sled motor 11 for carrying the pickup head and the suspension device like the tracking coil 12. From the point of view of control, the plant, that is, the controlled object is belonging to a bi-enabler and a system of bi-mass with low rigidity. The plant is the pickup head. The bi-enabler 112 refers to both the sled motor 11 and the tracking coil 12, and the system of bi-mass means that the lens 10 should be driven by sled motor 11 to move together. This motion of the bi-mass can induce the fluctuation of the position of the lens
During the operation of seek, the fluctuation of the lens 10 and the friction of the sled motor 11 itself could cause the fail of tracking control due to over speed after completing the operation of seek and switching to the following-track mode. Referring to the FIG. 2, in the conventional applications of optical disk drive, the method to yield the signals has taken advantages of the laser diode of pickup head to emit the laser beam which produces a Focusing Point on the optical disk and then is reflected back to the photo sensor 13 on the pickup head. After receiving the reflected laser beam, the photo sensor 13 yields six signals as A, B, C, D, E, and F for producing signals of tracking error (TE), focusing error (FE), and radio frequency (RF) etc., of which the signals are treated as the input of a control system 14 to yield the signal for doing the track crossing.
FIG. 3 illustrates the flow diagrams of a conventional velocity control. The operation of seek in a conventional optical disk drive intuitively is a control system of position, but during the control operation, the sled motor is continuously accelerated and results in the continuous fluctuation of the pickup head. This means that the position is not easy to be located. In other word, the operation of seek becomes more difficult. Therefore in general a control system of velocity is taken to assist the operation of seek.
According to the conventional method, a velocity profile 20 is designed first and the sled motor is accelerated or decelerated according to the velocity profile 20 so that the sled motor can be more smoothly accelerated or decelerated to reduce the fluctuation of pickup head. The flow of the velocity control is done through an adder which is receiving the outputs of the velocity profile 20 and a velocity estimator from the plant 22 and exporting an output for a velocity controller 21. The "+" and "-" mean the plus and subtraction of the indicated quantities, respectively. Here, there is a problem of steady state error still existing for the velocity control. As shown in FIG. 3, when the velocity controller C(s) 21 is a scalar k, the steady state error of the input I(s) in Laplace's operation can be expressed as ##EQU1##
Assume that the transfer function P(s) of the plant 22 as shown in FIG. 3 can be expressed as ##EQU2## where J represents the inertia moment, b represents viscous friction coefficient, and a represents the DC gain constant of the plant 22. If the input is a step function then I(s) can be expressed as ##EQU3## where R represents the amplitude. Then the steady state error can be expressed as ##EQU4## If the input is a ramp function, then I(s) can be expressed as ##EQU5## Then when the time has approached to infinite, the steady state error is expressed as ##EQU6##
The description above is under the case that the viscous friction coefficient is taken into account, If the viscous friction coefficient approaches to zero or small (b.fwdarw.0), then equation (4) (step function input) is zero and equation (6) (ramp function input) has a finite result as ##EQU7##
As the derivation described above, if the input function corresponding to the velocity profile in FIG. 4 is taken, then no matter whether the viscous friction coefficient does exit or not the error of control does always exist. FIG. 4 is an ideal velocity profile. After a certain time of operation, the steady state error is inevitable even though it isn't infinite as the result of equation (6). This steady state error E causes that the pickup head completes the operation not obeying the design of velocity profile Vc but obeying a similar velocity profile Vn as shown in FIG. 5, and then results in a fall of tracking control due to over speed while the control of the operation of seek does switch to the mode of following-track.
Summarizing the descriptions above, the conventional technology has some drawbacks as follows:
1. While the pickup head does the operation of seek for positioning, the conventional method as the position control would induce the fluctuation of the lens suspending on the pickup head and increase the difficulty of the control because the method can not directly control the acceleration and deceleration of the sled motor which is carrying and holding the pickup head.
2. If the conventional method of the velocity control is applied, even though the variation of velocity is able to be controlled to smooth the fluctuation of lens, the steady state error is still a problem and cause an increase of the fail rate of tracking control due to the over speed of the pickup head, while the control has been switched to the mode of following-track after reaching on the track number after the operation of seek.